Welding apparatus for resistance welding heat exchanger tube to tubesheet

ABSTRACT

Welding apparatus includes a first electrode having an electrode surface adapted to contact an outward end fold of a heat-exchanger tube which is positioned in an orifice of a tubesheet with the fold contacting an outward-facing side of the tubesheet. A second electrode has an electrode surface adapted to contact the outward-facing side of the tubesheet. Another welding apparatus includes a first electrode having an electrode surface adapted to contact an end of a heat exchanger tube positioned in an orifice of a tubesheet. The first electrode is movable to create an outward end fold in the tube and deform the fold against the outward-facing side of the tubesheet. Another welding apparatus includes a second electrode having an electrode surface adapted to contact the outward-facing side of a tubesheet and having cutouts which surround each of a plurality of heat-exchanger tubes and which are adapted to receive a first electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority of U.S. Provisional ApplicationNo. 60/585,966 filed Jul. 7, 2004.

TECHNICAL FIELD

The present invention relates generally to welding, and moreparticularly to a welding apparatus for resistance welding a heatexchanger tube to a tubesheet.

BACKGROUND OF THE INVENTION

Conventional methods for welding a tube to a tubesheet include gas metalarc welding and gas tungsten are welding. Gas metal arc welding uses aconsumable metal wire as one electrode and the parts as anotherelectrode, and moves the consumable metal wire (or the parts) to draw anarc and weld the parts together. Gas tungsten arc welding uses anon-consumable tungsten alloy electrode to draw an arc and a consumablefiller metal wire that is fed into the arc to weld parts together. Thewelding is accompanied by a gas (such as a mixture of argon and carbondioxide) to prevent oxidation and stabilize the arc. Such gas metal/gastungsten arc welding is well known. In a conventional gas metal arcwelding technique, solid metal wire or metal core wire (i.e., anannular-solid wire whose core is filled with metal powder such as amixture of metal, alloy and/or oxide powders) is used with the wiretypically at a positive electrical welding potential and with the partselectrically grounded. The welding arc creates a molten weld puddlewhich results in the welding together of the parts. Gas metal arcwelding requires expensive welding equipment, the molten weld puddletends to flow away from the joint area (depending on the joint positionwith respect to gravity) resulting in welds of inconsistent quality, andthe process requires a long cycle time between welds. The need toprecisely position the electrode at the joint and rotate around the tubecircumference precisely so that the arc is drawn at the location to bewelded is also difficult and time-consuming considering that thousandsof tubes may be welded to a tubesheet to from a heat-exchanger. Any lackof precision in positioning the electrode at the joint results inleakers in the heat-exchanger.

Conventional methods for attaching parts together also include frictionwelding. To join a tube to a plate, the tube is rotated about itslongitudinal axis, and the tube end and the plate are pressed together,wherein friction causes heating creating the weld. Friction weldingrequires expensive welding equipment, and the process requires a longcycle time between welds. Friction welding is not easily applicable tothin-walled tubes because they do not retain their shapes well underheat and pressure. Friction welding is not easily applicable to themanufacture of heat-exchangers, since the tubes are not commonlyrotatable after assembly for welding. It is noted that laser andelectron-beam welding for the above joints also need expensive equipmentand there is the need for precisely positioning and rotating the heatsource at the multitude of joints in a heat-exchanger.

What is needed is an improved welding apparatus for welding a heatexchanger tube to a tubesheet.

SUMMARY OF THE INVENTION

A first embodiment of the invention is for a welding apparatus whichincludes a resistance-welding first electrode and a resistance-weldingsecond electrode. The first electrode has an electrode surface adaptedto contact an outward end fold of a heat-exchanger tube which ispositioned in an orifice of a heat-exchanger tubesheet with the outwardend fold contacting an outward-facing side of the tubesheet. The secondelectrode has an electrode surface adapted to contact the outward-facingside of the tubesheet.

A second embodiment of the invention is for a welding apparatus whichincludes a resistance-welding first electrode and a resistance-weldingsecond electrode. The first electrode has an electrode surface adaptedto contact an end of a heat exchanger tube disposed in an orifice of aheat-exchanger tubesheet. The second electrode has an electrode surfaceadapted to contact an outward-facing side of the tubesheet. The firstelectrode is movable, with respect to the second electrode, to create anoutward end fold in the tube and deform the outward end fold against theoutward-facing side of the tubesheet.

A third embodiment of the invention is for a welding apparatus includinga resistance-welding first electrode and a resistance-welding secondelectrode. The first electrode has an electrode surface adapted tocontact an end portion of at least one of a plurality of heat-exchangertubes which are disposed in corresponding orifices of a heat-exchangertubesheet wherein the end portion protrudes from an outward-facing sideof the tubesheet. The second electrode has an electrode surface adaptedto contact the outward-facing side of the tubesheet and has cutoutswhich surround each of the plurality of tubes and which are adapted toreceive the first electrode.

Several benefits and advantages are derived from one or more of theembodiments of the invention. Welding using electric current is lessexpensive than gas metal arc welding or friction welding. Welding usingelectric current also has a shorter cycle time between welds than gasmetal arc welding or friction welding. In one example, the firstelectrode moves to deform the first outward fold against the firsttubesheet, wherein such deformation resistance welding allows solidstate welds of dissimilar materials without the formation of brittleintermetallic compounds. In one variation, having the first electrodealso create the outward fold streamlines the welding process. Havingresistance-welding first and second electrodes adapted to contact,respectively, the tube and the tubesheet on the same side of thetubesheet provides for an efficient welding process, as can beappreciated by those skilled in the art.

SUMMARY OF THE DRAWINGS

FIG. 1 is a block diagram of a first method of the invention;

FIG. 2 is a schematic side-elevational view of a first embodiment of avertically-positioned heat exchanger which includes a plurality oftubes, first and second tubesheets, and three baffles, and which can beassembled in accordance with one example of the first and/or secondmethod of the invention;

FIG. 3 is a schematic cross sectional view of a first embodiment offirst and second electrodes which can be used in welding aheat-exchanger tube to the first and second tubesheets in accordancewith one example of the first method of the invention;

FIG. 4 is a block diagram of a second method of the invention;

FIG. 5 is a schematic cross sectional view of a second embodiment offirst and second electrodes which can be used in welding aheat-exchanger tube to the first and second tubesheets in accordancewith one example of the second method of the invention;

FIG. 6 is a view, as in FIG. 5, but at a later time during the oneexample of the second method of the invention;

FIG. 7 is an end view of the first and second electrodes of FIG. 3;

FIG. 8 is an end view of an alternate first electrode which can replacethe first electrode of FIG. 3;

FIG. 9 is a cross sectional view of an embodiment of a second electrodewhich has a plurality of cutouts each surrounding a corresponding one ofa plurality of tubes;

FIG. 10 is a cross sectional view of an embodiment of first and secondelectrodes which can be used to simultaneously weld a plurality ofheat-exchanger tubes to a tubesheet in accordance with one elaborationof the first method; and

FIG. 11 is a cross sectional view of an embodiment of first and secondelectrodes which can be used to simultaneously weld a plurality ofheat-exchanger tubes to a tubesheet in accordance with one elaborationof the second method.

DETAILED DESCRIPTION

Referring to FIGS. 1-3, a first method of the invention is for weldingand includes steps a) through f). Step a) is labeled as “ObtainHeat-Exchanger Tube” in block 10 of FIG. 1. Step a) includes obtaining aheat-exchanger tube 12 having first and second tube ends 14 and 16 andhaving a first outward fold 18 disposed toward the first tube end 14.Step b) is labeled as “Obtain Tubesheet” in block 20 of FIG. 1. Step b)includes obtaining a heat-exchanger first tubesheet 22 having anoutward-facing side 24 and an opposing inward-facing side 26 and havingan orifice 28. Step c) is labeled as “Insert Tube Into Tubesheet” inblock 30 of FIG. 1. Step c) includes inserting the tube 12 at leastpartially through the orifice 28 of the first tubesheet 22 with thefirst outward fold 18 contacting one of the outward-facing side 24 andthe inward-facing side 26 of the first tubesheet 22. Step d) is labeledas “Dispose First Electrode” in block 32 of FIG. 1. Step d) includesdisposing a first electrode 34 in contact with the first outward fold18. Step e) is labeled as “Dispose Second Electrode” in block 36 ofFIG. 1. Step e) includes disposing a second electrode 38 in contact withthe one of the outward-facing side 24 and the inward-facing side 26 ofthe first tubesheet 22. Step f) is labeled as “Resistance Weld Tube ToTubesheet” in block 40 of FIG. 1. Step f) includes using (i.e., passingelectric current between) the disposed first and second electrodes 34and 38 to resistance weld the first outward fold 18 of the tube 12 tothe first tubesheet 22.

Describing the first outward fold 18 as being disposed toward the firsttube end 14 means the first outward fold 18 is disposed closer to thefirst tube end 14 than to the second tube end 16. In one example, thefirst outward fold 18 is disposed proximate the first tube end 14 of thetube 12 as shown in FIG. 2. By proximate is meant that the distance fromthe first outward fold 18 to the first tube end 14 is less than tenpercent of the distance from the midpoint of the tube 12 to the firsttube end 14. In one variation, the first outward fold (such as 18′ oftube 12′) is disposed at the first tube end 14. In one modification, thefirst outward fold 18 is a completely annular fold of the tube wall ofthe tube 12. In the same or a different modification, the first outwardfold 18 is a substantially transversely-extending fold of the tube wallof the tube. Other shapes of the first outward fold of the tube (i.e.,of the tube wall of the tube) are left to the artisan.

Describing the first tubesheet 22 as having an outward-facing side 24and an inward facing side 26 is meant that outward-facing side 24 is theside of the first tubesheet 22 that will be facing away from themidpoint of the tube 12 after welding and that the inward-facing side 26is the side of the first tubesheet 22 that will be facing toward themidpoint of the tube 12 after welding. In one construction, the tube 12has a substantially circularly annular shape. In a differentconstruction, the tube has a substantially rectangular shape. Othershapes of the tube, including non-straight tubes, are left to theartisan.

In one enablement of the first method, step c) inserts the tube 12completely through the orifice 28 of the first tubesheet 22. In onevariation, the one of the outward-facing side 24 and the inward-facingside 26 of the first tubesheet 22 is the outward-facing side 24 of thefirst tubesheet 22. In this variation, the first outward fold 18 isdisposed in contact with the outward-facing side 24 of the firsttubesheet 22. In the different enablement and/or variation, the one ofthe outward-facing side 24 and the inward-facing side 26 of the firsttubesheet 22 is the inward-facing side 26 of the first tubesheet 22.Here, the first outward fold 18 is disposed in contact with theinward-facing side 26 of the first tubesheet 22.

In one implementation of the first method, step f) includes moving thefirst electrode 34 to deform the first outward fold 18 against the firsttubesheet 22. With such movement, step f) can be described asdeformation resistance welding the first outward fold 18 of the tube 12to the first tubesheet 22. In some implementations, such as heatexchangers for boilers, the first tubesheet 22 is thick enough for afixture (not shown) to simply immobilize the first tubesheet 22 duringdeformation. In other implementations, such as vehicle radiators, afixture supplying additional support on the opposing side of thetubesheet proximate the first electrode is employed as can beappreciated by the artisan. In one operation of the first method, a stop(not shown) is provided to limit the movement of the first electrode.

In a first extension of the first method, the first method also includessteps g) through k). Step g) includes obtaining a heat-exchanger secondtubesheet 42 having an outward-facing side 24 and an opposinginward-facing side 26 and having an orifice 28. Step h) includesinserting the tube 12 at least partially through the orifice 28 of thesecond tubesheet 42. Step i) includes creating a second outward fold 44in the tube 12 toward the second tube end 16 of the tube 12. Step j)includes disposing the second outward fold 44 in contact with one of theoutward-facing side 24 and the inward-facing side 26 of the secondtubesheet 42. Step k) includes, after steps g) through j), resistancewelding the second outward fold 44 of the tube 12 to the secondtubesheet 42.

In one employment of the first extension of the first method, the secondoutward fold 44 is created using tube hydroforming techniques, as isknown to those skilled in the art. In a different employment, laterdescribed in detail, the first electrode is shaped to create the secondoutward fold (and in one variation to also create the first outwardfold). Other techniques for creating the second outward fold 44 are leftto the artisan.

In one application of the first extension of the first method, step h)inserts the tube 12 completely through the orifice 28 of the secondtubesheet 42. In the same or a different application, step k) includesdisposing the first electrode 34 in contact with the second outward fold44 and disposing the second electrode 38 in contact with the one of theoutward-facing side 24 and the inward-facing side 26 of the secondtubesheet 42. In the same or a different application, the firstextension of the first method also includes, before steps h) through k),the steps of obtaining a baffle 46 having a through hole 48 andinserting the tube 12 completely through the through hole 48. In oneutilization, several baffles 46 are used and act to reduce tubevibration.

Referring to FIGS. 2 and 4-6, a second method of the invention is forwelding and includes steps a) through g). Step a) is labeled as “ObtainHeat-Exchanger Tube” in block 50 of FIG. 4. Step a) includes obtaining aheat-exchanger tube 12′ having first and second tube ends 14 and 16.Step b) is labeled as “Obtain Tubesheet” in block 52 of FIG. 4. Step b)includes obtaining a heat-exchanger first tubesheet 22 having anoutward-facing side 24 and an opposing inward-facing side 26 and havingan orifice 28. Step c) is labeled as “Insert Tube Into Tubesheet” inblock 54 of FIG. 4. Step c) includes inserting the tube 12′ at leastpartially through the orifice 28 of the first tubesheet 22. Step d) islabeled as “Dispose First Electrode” in block 56 of FIG. 4. Step d)includes disposing a first electrode 58 in contact with the first tubeend 14 of the tube 12′. Step e) is labeled as “Dispose Second Electrode”in block 60 of FIG. 4. Step e) includes disposing a second electrode 62in contact with one of the outward-facing side 24 and the inward-facingside 26 of the first tubesheet 22. Step f) is labeled as “Create OutwardFold In Tube” in block 64 of FIG. 4. Step f) includes moving thedisposed first electrode 58 against the first tube end 14 to create afirst outward fold 18′ in the tube 12′ toward the first tube end 14 withthe first outward fold 18′ contacting the one of the outward-facing side24 and the inward-facing side 26 of the first tubesheet 22. Step g) islabeled as “Resistance Weld Tube To Tubesheet” in block 66 of FIG. 4.Step g) includes using the disposed first and second electrodes 58 and62 to resistance weld the first outward fold 18′ of the tube 12′ to thefirst tubesheet 22.

It is noted that the enablements, implementations, extensions,applications, etc. of the previously-described first method are equallyapplicable to the second method. In one example of an extension of thesecond method, the step of creating a second outward fold 44′ includesmoving the first electrode 58 against the second tube end 16 to createthe second outward fold 44′ in the tube 12′. In one utilization of thesecond method, one or more fixtures (not shown) are provided to supportthe tube 12′ and the first and second tubesheets 22 and 42.

Referring to FIGS. 2-3 and 7, a first embodiment of the invention is fora welding apparatus 68 which includes a resistance-welding firstelectrode 34 and a resistance-welding second electrode 38. The firstelectrode 34 has an electrode surface 70 adapted to contact an outwardend fold 18 of a heat-exchanger tube 12 which is disposed in an orifice28 of a heat-exchanger tubesheet 22 with the outward fold 18 contactingan outward-facing side 24 of the tubesheet 22. The second electrode 38has an electrode surface 72 adapted to contact the outward-facing side24 of the tubesheet 22.

In one enablement of the embodiment of FIGS. 2-3 and 7, the firstelectrode 34 is movable, with respect to the second electrode 38, todeform the contacted outward end fold 18 against the tubesheet 22. Inthis enablement, the first and second electrodes 34 and 38 are employedto deformation resistance weld the outward fold 18 of the tube 12 to thetubesheet 22.

In one deployment of the embodiment of FIGS. 2-3 and 7, wherein theoutward end fold 18 is an annular outward end fold 18, the electrodesurface 70 of the first electrode 34 is an annular electrode surface 70(as shown in FIG. 7) adapted to make complete annular contact with theannular outward end fold 18 of the tube 12. In a different deploymentwith an alternate first embodiment of the first electrode 33 shown inFIG. 8, wherein the outward end fold 18 is an annular outward end fold18, the first electrode 33 has annularly spaced-apart surface segments69 adapted to make segmented annular contact with the annular outwardend fold 18 of the tube 12.

In one configuration of the embodiment of FIGS. 2-3 and 7, the electrodesurface 70 of the first electrode 34 is a circularly annular electrodesurface 70, and the electrode surface 72 of the second electrode 38 is apartially or completely annular electrode surface 72 which has acircular inside diameter and which is radially outwardly spaced apartfrom the circularly annular electrode surface 70 of the first electrode34. In one variation, the circularly annular electrode surface 70 of thefirst electrode 34 is movable, with respect to the annular electrodesurface 72 of the second electrode 38, to deform the contacted outwardend fold 18 against the tubesheet 22. In one modification, the annularelectrode surface 72 of the second electrode 38 has a fluted outerperimeter to avoid contact with any adjacent additional heat-exchangertube 12. In one employment, the first electrode 34 welds one tube 12 ata time to the tubesheet 22, and the second electrode 38 surrounds onlythe one tube 12 at a time. In one arrangement, not shown, where tubesare spaced closely together, the first electrode includes a plurality offirst sub-electrodes and/or the second electrode includes a plurality ofsecond sub-electrodes, wherein resistance-welding current flows betweenthe first and second sub-electrodes.

Referring to FIGS. 2 and 5-6, a second embodiment of the invention isfor a welding apparatus 74 which includes a resistance-welding firstelectrode 58 and a resistance-welding second electrode 62. The firstelectrode 58 has an electrode surface 76 adapted to contact a tube end14 of a heat exchanger tube 12′ disposed in an orifice 28 of aheat-exchanger tubesheet 22. The second electrode 62 has an electrodesurface 78 adapted to contact an outward-facing side 24 of the tubesheet22, wherein the first electrode 58 is movable, with respect to thesecond electrode 62, to create an outward end fold 18′ in the tube 12′and deform the outward end fold 18′ against the outward-facing side 24of the tubesheet 22.

In one enablement of the embodiment of FIGS. 2 and 5-6, the outward endfold 18′ is an annular outward end fold 18′, and the electrode surface76 of the first electrode 58 includes an annular ledge 80 adapted tomake complete annular contact with the annular outward end fold 18′ ofthe tube 12′. In one variation, the first electrode 58 includes anelectrode end 82, and the electrode surface 76 of the first electrode 58includes a tapered portion 84 which extends from the annular ledge 80 tothe electrode end 82. In one modification, the tapered portion 84 is aconical portion 86. In one configuration, the first electrode 58 has alongitudinal axis 88, and the annular ledge 80 is asubstantially-circular annular ledge 80 which is substantiallyperpendicular to the longitudinal axis 88. In one arrangement, theconical portion 86 is a truncated conical portion 86. In one usage, thefirst electrode 58 is moved such that the truncated conical portion 86of the electrode surface 76 of the first electrode 58 bends the tube end14 of the tube 12′ as shown in FIG. 5, and such that thereafter thesubstantially-circular annular ledge 80 bends the tube end 14 of thetube 12′ as shown in FIG. 6 creating the annular outward end fold 18′.

Another embodiment of a second electrode 90 is shown together with atubesheet 22 in FIG. 9. A third embodiment of a welding apparatus of theinvention includes a first electrode combination which combines thefirst electrode 34 of the embodiment of FIGS. 2-3 and 7 and the secondelectrode 90 of FIG. 9. In the first electrode combination, the weldingapparatus includes a resistance-welding first electrode 34 and aresistance-welding second electrode 90. The first electrode 34 has anelectrode surface 70 adapted to contact an end portion 92 of at leastone of a plurality of heat-exchanger tubes 12 which are disposed incorresponding orifices 28 of a heat-exchanger tubesheet 22 wherein theend portion 92 protrudes from an outward-facing side 24 of the tubesheet22. The second electrode 90 has an electrode surface 94 adapted tocontact the outward-facing side 24 of the tubesheet 22 and has cutouts96 which surround each of the plurality of tubes 12 and which areadapted to receive the first electrode 34.

In one employment of the first electrode combination, the end portion 92includes an outward end fold 18, and the electrode surface 70 of thefirst electrode 34 is adapted to contact the outward end fold 18 of theat-least-one tube 12. In one variation, the electrode surface 70 of thefirst electrode 34 is adapted to contact only one at a time the outwardend fold 18 of each of the plurality of tubes 12, and the firstelectrode 34 is movable, with respect to the second electrode 38, todeform the contacted outward end fold 18 against the tubesheet 22. In adifferent variation, shown in FIG. 10, the electrode surface 71 of thefirst electrode 35 is adapted to contact substantially simultaneouslythe outward end fold 18 of each of the plurality of tubes 12, and thefirst electrode 35 is movable, with respect to the second electrode 90,to deform the contacted outward end folds 18 against the tubesheet 22.The later variation can be visualized as a first electrode 35 having atleast two first electrode portions movable together as a unit, with eachfirst electrode portion being a first electrode 34 shown in FIGS. 3 and7. In one design, not shown, the second electrode includes two secondelectrode portions disposed in contact with each other at an overlappingjoint.

A fourth embodiment of a welding apparatus of the invention includes asecond electrode combination which combines the first electrode 58 ofthe embodiment of FIGS. 2 and 5-6 and the second electrode 90 of FIG. 9.It is noted that the end portion 92′ of the tube 12′ includes a tube end14. In the second electrode combination, the welding apparatus includesa resistance-welding first electrode 58 and a resistance-welding secondelectrode 90. The electrode surface 76 of the first electrode 58 isadapted to contact the tube end 14 of the at-least-one tube 12′, whereinthe first electrode 58 is movable, with respect to the second electrode62, to create an outward end fold 18′ in the at-least-one tube 12′ anddeform the outward end fold 18′ of the at-least-one tube 12′ against anoutward-facing side 24 of the tubesheet 22. The second electrode 90 hasan electrode surface 94 adapted to contact the outward-facing side 24 ofthe tubesheet 22 and has cutouts 96 which surround each of the pluralityof tubes 12′ and which are adapted to receive the first electrode 34.

In one variation of the second electrode combination, the electrodesurface 76 of the first electrode 58 is adapted to contact only one at atime the tube end 14 of each of the plurality of tubes 12′. In adifferent variation, shown in FIG. 11, the electrode surface 77 of thefirst electrode 59 is adapted to contact substantially simultaneouslythe tube end 14 of each of the plurality of tubes 12′. The latervariation can be visualized as a first electrode 59 having at least twofirst electrode portions movable together as a unit, with each firstelectrode portion being a first electrode 58 shown in FIGS. 5-6.

Several benefits and advantages are derived from one or more of theembodiments of the invention. Welding using electric current is lessexpensive than gas metal arc welding or friction welding. Welding usingelectric current also has a shorter cycle time between welds than gasmetal arc welding or friction welding. In one example, the firstelectrode moves to deform the first outward fold against the firsttubesheet, wherein such deformation resistance welding allows solidstate welds of dissimilar materials without the formation of brittleintermetallic compounds. In one variation, having the first electrodealso create the outward fold streamlines the welding process. Havingresistance-welding first and second electrodes adapted to contact,respectively, the tube and the tubesheet on the same side of thetubesheet provides for an efficient welding process, as can beappreciated by those skilled in the art.

The foregoing description of several methods and embodiments of theinvention has been presented for purposes of illustration. It is notintended to be exhaustive or to limit the invention to the preciseprocedures or precise forms disclosed, and obviously many modificationsand variations are possible in light of the above teaching. It isintended that the scope of the invention be defined by the claimsappended hereto.

1. A welding apparatus comprising: a) a resistance-welding firstelectrode having an electrode surface adapted to contact an outward endfold of a heat-exchanger tube which is disposed in an orifice of aheat-exchanger tubesheet with the outward end fold contacting anoutward-facing side of the tubesheet; and c) a resistance-welding secondelectrode having an electrode surface adapted to contact theoutward-facing side of the tubesheet.
 2. The welding apparatus of claim1, wherein the first electrode is movable, with respect to the secondelectrode, to deform the contacted outward end fold against thetubesheet.
 3. The welding apparatus of claim 1, wherein the outward endfold is an annular outward end fold, and wherein the electrode surfaceof the first electrode is an annular electrode surface adapted to makecomplete annular contact with the annular outward end fold of the tube.4. The welding apparatus of claim 1, wherein the outward end fold is anannular outward end fold, and wherein the electrode surface of the firstelectrode has annularly spaced-apart electrode surface segments adaptedto make segmented annular contact with the annular outward end fold ofthe tube.
 5. The welding apparatus of claim 1, wherein the electrodesurface of the first electrode is a circularly annular electrodesurface, and wherein the electrode surface of the second electrode is apartially or completely annular electrode surface which has a circularinside diameter and which is radially outwardly spaced apart from thecircularly annular electrode surface of the first electrode.
 6. Thewelding apparatus of claim 5, wherein the circularly annular electrodesurface of the first electrode is movable, with respect to the annularelectrode surface of the second electrode, to deform the contactedoutward end fold against the tubesheet.
 7. The welding apparatus ofclaim 5, wherein the annular electrode surface of the second electrodehas a fluted outer perimeter to avoid contact with any adjacentadditional heat-exchanger tube.
 8. A welding apparatus comprising: a) aresistance-welding first electrode having an electrode surface adaptedto contact a tube end of a heat exchanger tube disposed in an orifice ofa heat-exchanger tubesheet; and b) a resistance-welding second electrodehaving an electrode surface adapted to contact an outward-facing side ofthe tubesheet, wherein the first electrode is movable, with respect tothe second electrode, to create an outward end fold in the tube anddeform the outward end fold against the outward-facing side of thetubesheet.
 9. The welding apparatus of claim 8, wherein the outward endfold is an annular outward end fold, and wherein the electrode surfaceof the first electrode includes an annular ledge adapted to makecomplete annular contact with the annular outward end fold of the tube.10. The welding apparatus of claim 9, wherein the first electrodeincludes an electrode end, and wherein the electrode surface of thefirst electrode includes a tapered portion which extends from theannular ledge to the electrode end.
 11. The welding apparatus of claim10, wherein the tapered portion is a conical portion.
 12. The weldingapparatus of claim 11, wherein the first electrode has a longitudinalaxis, and wherein the annular ledge is a substantially-circular annularledge which is substantially perpendicular to the longitudinal axis. 13.The welding apparatus of claim 12, wherein the conical portion is atruncated conical portion.
 14. A welding apparatus comprising: a) aresistance-welding first electrode having an electrode surface adaptedto contact an end portion of at least one of a plurality ofheat-exchanger tubes which are disposed in corresponding orifices of aheat-exchanger tubesheet wherein the end portion protrudes from anoutward-facing side of the tubesheet; and b) a resistance-welding secondelectrode having an electrode surface adapted to contact theoutward-facing side of the tubesheet and having cutouts which surroundeach of the plurality of tubes and which are adapted to receive thefirst electrode.
 15. The welding apparatus of claim 14, wherein the endportion includes an outward end fold, and wherein the electrode surfaceof the first electrode is adapted to contact the outward end fold of theat-least-one tube.
 16. The welding apparatus of claim 15, wherein theelectrode surface of the first electrode is adapted to contact only oneat a time the outward end fold of each of the plurality of tubes, andwherein the first electrode is movable, with respect to the secondelectrode, to deform the contacted outward end fold against thetubesheet.
 17. The welding apparatus of claim 15, wherein the electrodesurface of the first electrode is adapted to contact substantiallysimultaneously the outward end fold of each of the plurality of tubes,and wherein the first electrode is movable, with respect to the secondelectrode, to deform the contacted outward end folds against thetubesheet.
 18. The welding apparatus of claim 14, wherein the endportion includes a tube end, and wherein the electrode surface of thefirst electrode is adapted to contact the tube end of the at-least-onetube, wherein the first electrode is movable, with respect to the secondelectrode to create an outward end fold in the at-least-one tube anddeform the outward end fold of the at-least-one tube against theoutward-facing side of the tubesheet.
 19. The welding apparatus of claim18, wherein the electrode surface of the first electrode is adapted tocontact only one at a time the tube end of each of the plurality oftubes.
 20. The welding apparatus of claim 18, wherein the electrodesurface of the first electrode is adapted to contact substantiallysimultaneously the tube end of each of the plurality of tubes.